frequently asked questions:

1) What does the planting material consist of here in Ohio?Depending on the owner's main objectives, there are plants to choose for higher transpiration/air cooling rates, or there are specific plants to choose if color and aesthetics are your objective.Sedums, like stonecrop, are typically used because they of their drought tolerance and transpiration rate. Other local nurseries are using herbs and native perennials like Tickseed, Dianthus, Catmint. Click HERE to download a printable map and plant list used at Howlett Hall.

2) What happens to the planting material in Ohio's cold months, and how does that effect buildings internal temperature? The plants go dormant in the winter, therefore the greenroof goes dormant. It still functions in terms of added insulation and roof membrane protection from harsh winter elements, however due to low transpiration rates in the winter, it is not as effective in terms of stormwater mitigation.

3) Why would green roofs be a good investment for The Ohio State University moving forward? There are many benefits of a green roof - storm water runoff mitigation, thermal insulation, heat island effect reduction, increased biodiversity, increasing carbon sequestration, and improved aesthetics are among the most common.

In Columbus, OH where we had more than 7 inches of rain in April 2011, a green roof would mitigate nearly 80% of that stormwater runoff and prevent overburdening the city’s sewer system and avoiding further pollution of our rivers and streams. With an average annual rainfall of 37 inches/year in Columbus, our green roof would mitigate a minimum 22.8 inches (60% mitigation) compared to 95% runoff coefficient of the surrounding parking lots. The projected green roof coverage will reduce the impact to the watershed system by approximately 976,000 liters (approx 250,000 gallons) of water each year.

Extending the life expectancy of a roof- A conventional roof lasts 15-25 years but a greenroof can extend that lifetime to nearly double due to cooler temperatures, less fluctuations (conventional temps fluctuate up to 150˚/day), and protection from ultra violet rays.

Thermal insulation - The vegetation creates a more efficient system by absorbing heat from the building envelope to reduce thermal heat gain in the summer. Plants absorb and use rainfall through transpiration, the process by which plants take up water and ‘sweat’ it out to the atmosphere. Transpiration is one of the main consumers of solar energy at the Earth's surface and is a very beneficial function for the building. Projections show that the Green Roof at Howlett Hall will reduce the roof surface temperature of our south-facing building by nearly 90% and in turn reduce cooling costs by 25%-50%.

5. How many plants are on the roof garden? We covered about half of the roof with a pre-grown sedum mat, with 11-14 varieties of sedum. The other half was planted with 7225 perennial plants including sedum, Achillea (Yarrow), Rudbeckia (Black Eyed Susan), Festuca (Blue Fescue), Thyme, Lavender, Allium, Prairie Drop Seed, Geum and Sempervivum (Hens and Chicks).